Fiber optic links are quite useful for transferring high bandwidth data between components that may be quite distant from each other. Examples of these links include data communication trunks, intranets, and (even lately) flight control systems. With regard to the use of fiber aboard aircraft, and other mobile platforms, fiber optic technology allows large amounts of targeting, navigation, communication, command, and control data to be shared with the pilot thereby allowing the pilot increased situational awareness.
Generally, fiber technology provides a higher bandwidth capability than conventional, copper wire systems. However, fiber technology has been constrained in its use because the fiber optic transceivers currently available can transmit or receive data on one fiber, but can not do both on that one fiber without incurring significant performance penalties (e.g. loss of bandwidth or signal strength) or complicating the design of the transceivers and the overall system.
Thus, for every bidirectional communication need in which simplicity and performance are desired, the transceivers at the ends of the link must have one optical port for transmitting data signals and a second optical port for receiving data signals. Further, the duplication of parts (primarily optical fibers and connectors) extends along the length of the link. Each direction of the link therefore requires a complete set of cables and connectors when only one cable assembly is used with transceivers that transmit and receive optical signals from a single port.
If either performance or system simplicity can be sacrificed, then existing bidirectional fiber optic links that are more complex or have more loss can be used. These links fall into two categories. The first category of bidirectional link uses two different wavelengths (one for each direction) and dichroic mirrors (mirrors that transmit or reflect based on the wavelength) or, perhaps, a wavelength splitter to combine and split the signals traveling in the two directions with low loss. This approach has the disadvantages of requiring multiple wavelengths for the same link and of complicating the system configuration. More particularly, on one side of the link, the transmitter operates at a first wavelength with the receiver at the other end also operating at the first wavelength. For the return link, the transmitter operates at a second wavelength with the receiver operating at the second wavelength. Thus, for each bidirectional link, two pairs of transmitters and receivers must be supplied with each pair operating at separate distinct wavelengths.
The second type of bidirectional link uses free space or fiber beamsplitters at each end to manage (combine or separate) the incoming and outgoing optical signals. This type of link uses only a single wavelength, but it wastes half the available optical power at the transmitter (at one end of the link) and another half of the available optical power at the receiver (at the other end of the link). Thus, only 25% of the available transmitted power can be received at best. As a result, the signal to noise ratio, or the bandwidth, of the link decreases accordingly.